1. Field of the Invention
The present invention relates to prosthetic devices and, more specifically, to a system and method for replication of angular position of an articulating head of a prosthesis. The invention has particular application to the humeral component of a shoulder prosthesis.
2. Background Information
Repair and replacement of human joints, such as the knee, shoulder, elbow and hip, has become a more and more frequent medical treatment. Longer life spans mean that the joints endure more wear and tear. More sports activities mean greater likelihood of serious joint injuries. Treatment of injuries, wear and disease in human joints has progressed from the use of orthotics to mask the problem, to fusion of the joint, to the use of prostheses to replace the damaged joint component(s).
As the success rate for total or partial joint replacements has increased, so too has the need for modularity and universality in the joint prosthesis. Patient variety means that no single size or configuration of joint prosthesis will suffice. The physical dimensions of a patient's joint components vary, as well as the bio-mechanic relationship between these components. For instance, in a shoulder prosthesis, the relationship between the articulating humeral and glenoid components can be significantly different between patients. These relationships are especially important where only one component of the joint is being replaced and must integrate with the existing natural opposing joint component.
For instance, in many shoulder surgeries, only the humeral component is replaced, leaving the glenoid component intact. In this case, it is imperative that the articulating surface of the humeral component match the articulating surface of the glenoid component as perfectly as possible, both statically and dynamically. With a typical humeral prosthesis, version and inclination are adjusted by the geometry of the head of the prosthesis. In other words, certain pre-determined head geometries are available that can be selected for a mating glenoid component. Absent an infinite variety of pre-determined head geometries, the resulting humeral prosthesis can often only achieve a best-fit relationship to the glenoid component of the shoulder joint.
Presently, two strategies are available to a surgeon for shoulder replacement surgery. One strategy is to perform the shoulder replacement surgery in accordance with the design of a particular manufacturer's shoulder prosthesis or shoulder prosthesis product line. In this case, a surgeon is provided with instrumentation and technique guidelines for the particular shoulder prosthesis or prosthesis line. The guidelines and/or instrumentation direct or dictate the angle of humeral head resection for the implant (prosthesis). This angle is in relation to the humeral intramedullary canal and is designed to match an optimum set of angles already present in the design of the prosthesis.
Another approach is to perform the shoulder replacement surgery in accordance with the patient's anatomy. Particularly, the humeral head is resected according to angles perceived to be “anatomic” in the opinion of the surgeon, not according to angles already present in the prosthesis itself. With this approach, the prosthesis is designed so that its configuration is intraoperatively adjustable. This allows the prosthesis to be adjustable in situ so that it can match the bony preparation.
Even with respect to these two divergent manners of surgical strategy, a common problem in shoulder surgery is matching the humeral resection angle across the articular margin to the predetermined angle designed into the prosthesis. This angle may include the angle between a prosthetic collar and the diaphyseal section of the stem. In the case of a collarless stem, the angle may inscribe the difference between the longitudinal axis of the stem and the inferior surface of the prosthetic head. It is considered optimal for fixation and biomechanics if the resected angle and the angle of the prosthesis are identical, thereby allowing intimate contact between the superior surface of the resected bone and the inferior surface of the implant.
Moreover, the angular version in which the prosthesis is implanted will have a significant impact on the biomechanics of the prosthetic joint. Many shoulder prosthesis systems on the market dictate the varus/valgus angle of the bone cut. This strategy does not allow the surgeon to intraoperatively match the implant to the patient's biomechanics after the prosthesis has been trialed, much less implanted. There are two known products currently marketed that attempt to resolve at least one of the above-noted issues. First, the Tornier-Aequalis system provides a modular junction within the metaphyseal region of the stem which allows a small block between the stem and humeral head to be interchanged. This block is available in multiple angles, thus allowing the surgeon to select the block that best fits the bony anatomy as resected. This system, however, has two primary weaknesses. First, the use of modular blocks obviously forces the design to only allow angular adjustments in finite increments. Second, the need to adjust the angle through modular blocks forces the surgeon to remove the stem, change out a component, and reset the stem.
A second product currently marketed provides a humeral head that is infinitely adjustable in varus/valgus and anterior/posterior angles relative to the stem portion of the prosthesis. This is accomplished through a spherical shaped protrusion on the superior surface of the stem that fits into a spherical recess in the humeral head. These mating surfaces allow the head to be articulated about the stem, thus allowing adjustable positioning of the head. The head can be locked in a position relative to the stem. This solution provides the ability to adjust the neck-shaft angle as well as the version through flexibility in the anterior/posterior angle. The locking mechanism, however, is sub-optimal since it requires the turning of a locking screw that has its head facing lateral and inferior, for which there is no access once the stem has been cemented. This eliminates the ability to adjust head position on the fly, and forces a total revision if articular surfaces ever need to be revised. Lastly, the protrusion on the humeral stem even when the humeral head is not in place limits the surgeon's access to the glenoid in preparation for a glenoid replacement.
An improvement to this latter product places an adjustable mounting element between the stem and the humeral head. The mounting element is configured for articulating engagement with the stem to permit angular positioning of the head component in multiple degrees of freedom. Details of this prosthesis are found in co-pending application Ser. No. 10/748,448 (the '448 application), entitled JOINT PROSTHESIS WITH INFINITELY POSITIONABLE HEAD, filed on Dec. 30, 2003, and owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference.
As disclosed in the '448 application, the humeral head is fixed to the mounting element by a press-fit engagement. The mounting element is fastened to the humeral stem by two mechanisms. In the first mechanism, the mounting element achieves a friction fit with a tapered bore in the neck of the humeral stem. The second fixation mechanism includes a screw that is threaded into a threaded bore portion of the tapered bore in the stem. The screw bears against the mounting element to lock the element in position within the tapered bore. The joint prosthesis in this '448 application is both modular and universal in that it permits infinitely variable positioning of a mating joint component relative to a bone engaging portion of the prosthesis. Moreover, this improved prosthesis is readily available for modification, whether during initial implantation or during a subsequent revision procedure.
With shoulder prostheses that allow a surgeon to adjust the angular position of the humeral head, such as those described above, a method must be available for trialing the prosthesis. When the trial prosthesis is implanted, several adjustments may be made to set the angular position of the prosthetic head relative to the humeral stem. In a typical trialing system, the trial prosthesis includes a broach configured to be tightly received within a previously prepared intramedullary (IM) canal of the humerus. In current systems, an articulating element is oriented relative to the neck of the broach and locked in place by a press-fit taper. Locking the articulating trial element thus requires impaction of the element within the broach. This method produces galling of the broach which can significantly limit the useful life of the broach. Moreover, the impaction step frequently causes the trial broach to sink further into the IM canal. This displacement of the trial broach results in an indeterminate offset of the center of rotation of the trial element. In addition, discrepancies between the amount of impaction of the trial element vis-à-vis the final implant element results in an unknown offset of this center of rotation, which ultimately leads to a poor anatomic fit and improper alignment of the humeral head prosthesis.
There is a need for a trialing system that avoids these problems of the current trialing approaches. There is a further need for a trialing system that can ensure accurate duplication of the angles of the trial implant without using impaction to fix the trial components.